Sixth seeded Czech Safarova was down a set to American Riske and trailed 5-3 in the second before fighting back to take the match into a third set.

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Girls' champion knocks out ninth seed

Williams, meanwhile, is the favourite for the women's title.

Should she triumph at SW19 for a sixth time, the top seed will move closer to completing a calendar Grand Slam and emulate Steffi Graf, who won all four titles in 1988.

But to the surprise of many, 20-year-old Russian Gasparyan - 113 places below the world number one in the rankings - broke the American at the start. Parity was restored in the sixth game, when Williams seized on her second break point of the game to level at 3-3, her opponent's backhand volley drifting wide.

Although Gasparyan was not initially overawed, confidently hitting clean winners, Williams eventually overpowered her opponent as the experience of a woman who has lost just one of her 59 first-round matches in Grand Slams told.

Williams went 3-0 ahead in the second set and when she saved three break points in the fifth game for a 4-1 lead, victory felt like an inevitability.

The win, her 73rd singles victory at Wimbledon to draw level with sister Venus, was achieved with a vicious overhead smash on her second match point.

The American will next play Timea Babos of Hungary.

Venus in a rush

Venus Williams won Wimbledon in 2000, 2001, 2005, 2007 and 2008

Venus Williams joined her sister in the second round with a comfortable 6-0 6-0 win against fellow American Madison Brengle.

The 25-year-old could not handle Venus's power as the five-time champion breezed to victory in just 42 minutes.

The win keeps alive the prospect of Venus meeting Serena in the fourth round.

"It would be great if I can continue to play well and get there, play next week. That would be great," said Venus, who has played four Wimbledon finals against her sister.